1. Field of the Invention
In general, the present invention relates to a method of producing a lithographic mask (reticle) for use in the semiconductor industry. In particular, the invention pertains to a method of producing a patterned photoresist used in preparation of the reticle. The optical imaging of the photoresist makes use of a deep ultraviolet (DUV) radiation. The DUV photoresist is imaged using an optical direct write continuous laser mask writing tool.
2. Brief Description of the Background Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components, such as in the fabrication of semiconductor device structures. The miniaturized electronic device structure patterns are typically created by transferring a pattern from a patterned masking layer overlying the semiconductor substrate. The pattern is typically created in the semiconductor substrate masking layer by a process which includes blanket radiation through a reticle to produce an image of the pattern in the semiconductor substrate masking layer. A reticle is a patterned mask which is used in combination with an optical imaging tool to produce the pattern in the semiconductor substrate patterned masking layer. Blanket irradiation through a reticle is used to pattern the semiconductor substrate rather than direct writing of radiation upon the semiconductor substrate masking layer because of the time economy which can be achieved by blanket irradiation of the pattern through a reticle. A direct write process typically requires from about 8 hours to about 20 hours, while blanket irradiation through a reticle typically requires less than a minute, and often seconds. With regard to semiconductor device processing, the semiconductor substrate masking layer may be a patterned photoresist layer or may be a patterned “hard” masking layer (typically an inorganic material or a high temperature organic material) which resides on the surface of the semiconductor device structure to be patterned.
A reticle typically includes a thin metal-containing layer (such as a chrome-containing, molybdenum-containing, or tungsten-containing material, for example) deposited on a glass or quartz plate. The reticle is patterned to contain a “hard copy” of the individual device structure pattern to be recreated on the semiconductor substrate masking layer overlying the semiconductor device structure. The reticle may be created by a number of different techniques, depending on the method of writing the pattern on the reticle. Due to the dimensional requirements of today's semiconductor structures, the writing method is generally with a laser or e-beam. A typical process for forming a reticle may include: providing a glass or quartz plate, depositing a chrome-containing layer on the glass or quartz surface, depositing an antireflective coating (ARC) over the chrome-containing layer, applying a photoresist layer over the ARC layer, direct writing on the photoresist layer to form a desired pattern, developing the pattern in the photoresist layer, etching the pattern into the chrome layer, and removing the residual photoresist layer. When the area of the photoresist layer contacted by the writing radiation becomes easier to remove during development, the photoresist is referred to as a positive-working photoresist. When the area of the photoresist layer contacted by the writing radiation becomes more difficult to remove during development, the photoresist is referred to as a negative-working photoresist. Advanced reticle manufacturing materials frequently include combinations of layers of materials selected from chromium, chromium oxide, chromium oxynitride, molybdenum, molybdenum silicide, and molybdenum tungsten silicide, for example.
As previously mentioned, the reticle or photomask is used to transfer a pattern to an underlying semiconductor substrate masking layer, which is often referred to as a photoresist. Blanket irradiation passes through open areas of the reticle onto the surface of the photoresist. The photoresist is then developed and used to transfer the pattern to an underlying semiconductor structure. Due to present day pattern dimensional requirements, which are commonly less than 0.25 Φm, the photoresist is preferably a chemically amplified DUV photoresist.
A chemically amplified DUV photoresist material is commonly used in creating the reticle as well, where the pattern image in the photoresist material is created using a direct write electron beam writing tool or an optical direct write laser. Optical direct writing laser tools are available under the trade name ALTA™ from ETEC Systems Inc., Hillsboro, Oreg.
Preparation of a photomask/reticle is a complicated process involving a number of interrelated steps which affect the critical dimensions of a pattern produced in the reticle, and the uniformity of the pattern critical dimensions across the surface area of the reticle. By changing various steps in the reticle manufacturing process, the reproducibility of the manufacturing process itself may be altered, including the processing window. Processing window refers to the amount process conditions can be varied without having a detrimental outcome on the product produced. The larger the processing window, the greater change permitted in processing conditions without a detrimental affect on the product. Thus, a larger process window is desirable, as this generally results in a higher yield of in specification-compliant product produced.
The reticle manufacturing process steps generally include the following, where the initial substrate used to form the reticle is a silicon oxide-containing base layer having a layer of a metal-containing (typically chrome) mask material applied thereover. An inorganic antireflective coating (ARC) or an organic ARC, or a combination of inorganic and organic ARC layers may be applied over the surface of the chrome mask material. A photoresist layer is then applied over the antireflective coating. The photoresist is typically an organic material which is dissolved or dispersed in a solvent The solution or dispersion of photoresist is typically spin coated onto the surface of the photomask fabrication structure. Typically, the photoresist is applied over an ARC layer on the fabrication structure surface. Some of the solvent or dispersion medium is removed during the spin coating operation. Residual solvent or dispersion medium is subsequently removed by another means, typically by baking the fabrication structure, including the photoresist layer. This step is commonly referred to as “Post Apply Bake” or PAB. The photoresist is then exposed to radiation (imaged), to produce a pattern in the photoresist layer, typically by a direct write process when the pattern includes dimensions which are less than about 0.25 Φm or less. After exposure, the substrate including the photoresist layer is baked again. The second baking is typically referred to as “Post Exposure Bake” or PEB. The photoresist is then developed either using a dry process or a wet process, to create a pattern having openings through the photoresist layer thickness. Once the photoresist is “patterned” so that the pattern openings extend through the photoresist layer to the upper surface of an ARC layer, or to a surface beneath an ARC layer, the pattern in the patterned photoresist is transferred through the chrome-based mask layer and any remaining layers overlying the chrome layer, for example, typically by dry etching.
U.S. Pat. No. 6,703,169, issued Mar. 9, 2004 to Fuller et al., and titled: “Method Of Preparing Optically Imaged High Performance Photomasks”, is assigned to the assignee of the present invention, and describes a method of producing a reticle via an optically imaged photoresist using a direct write continuous wave laser. In particular, the invention pertains to a method of optically fabricating a photomask using a direct write continuous wave laser, which includes the steps of applying an organic antireflection coating over a metal-containing layer; applying a chemically-amplified DUV photoresist, either positive tone or negative tone, over the organic antireflection coating; baking the DUV photoresist at a temperature within a specifically designed range under ambient conditions, with volatile removal assisted by an exhaust hood fan or by similar method (PAB); exposing a surface of the DUV photoresist to radiation from the direct write continuous wave laser; baking the developed photoresist at a temperature within a specifically designed range, again under ambient conditions using an exhausted hot plate (PEB); and, developing the image within the DUV photoresist. Preferably the laser used to image the DUV photoresist is operated at a wavelength between about 257 nm and about 198 nm, although other wavelengths may be used. Subsequently, the developed, patterned photoresist is used as a mask for transferring the pattern through a metal-containing layer of the photomask substrate. Typically the pattern transfer is by dry etch. The metal-containing layer of the photomask substrate may include combinations of layers of materials selected from chromium, chromium oxide, chromium oxynitride, molybdenum, molybdenum silicide, and molybdenum tungsten silicide, for example and not by way of limitation. This patent application is hereby incorporated by reference in its entirety.
U.S. Pat. No. 6,605,394, issued Aug. 12, 2003 to Montgomery et al., and titled: “Organic Bottom Antireflective Coating For High Performance Mask Making Using Optical Imaging”, is assigned to the assignee of the present invention, and describes a reticle fabrication process, with emphasis on the bottom ARC layers used beneath the photoresist, during patterning of the photoresist. One embodiment of the invention pertains to a method of optically fabricating a photomask using a direct write continuous wave laser, which includes the steps of applying an organic antireflection coating over a metal-containing layer; applying a chemically-amplified DUV photoresist, either positive tone or negative tone, over the organic antireflection coating; and exposing a surface of the DUV photoresist to radiation from the direct write continuous wave laser. Preferably the laser is operated at 244 nm or 257 nm. The metal-containing layer may include combinations of layers of materials selected from chromium, chromium oxide, chromium oxynitride, molybdenum, molybdenum silicide, and molybdenum tungsten silicide, for example and not by way of limitation. The organic antireflection coating may be selected from a negative photoresist containing a DUV dye; a polymeric material prepared from acrylic polymers or copolymers; a binder resin combined with an acid or thermal acid generator and a photoacid generator compound; a binder resin having pendent phenyl groups; and combinations thereof. The organic anti-reflective coating composition preferably comprises acrylic polymers and/or copolymers. In an alternative embodiment of the method of fabricating a photomask, the organic antireflection-coating is applied over an inorganic antireflection coating. The inorganic antireflection coating may be selected to include a material such as chrome oxynitride, titanium nitride, silicon nitride or molybdenum silicide. This patent application is hereby incorporated by reference in its entirety.
As disclosed in the '394 patent, there are a number of problems encountered in trying to produce a photomask/reticle when the photomask pattern exhibits critical dimensions of less than 0.25 Φm (250 nm). One of the more important problem areas is the process bias which occurs, in part, as a result of the wet development of the photoresist which is used to pattern the reticle. As the developed pattern dimensions have become smaller, portions of the pattern are dissolved away during the wet development, leading to a loss in resolution in the etched thin metal-containing layer of the reticle which is patterned to produce a “hard copy” of the individual device structure pattern to be recreated on the semiconductor substrate masking layer.
In an article entitled “Role of etch pattern fidelity in the printing of optical proximity corrected photomasks” by A. Komblit et al., published in J.Vac.Sci Technol. B 13(6), November/December 1995, the authors describe optical proximity effect correction (when isolated and dense features are both present in a pattern) which is said to improve critical dimension control, increase overlay margins, and when used with phase-shifting masks and/or modified illumination and mask-plane nonprinting assist features, to also extend resolution and increase depth of focus of current generation lithographic exposure technology. Optical proximity effect correction (OPC) is described as a methodology whereby the imaging mask is intentionally distorted in an effort to partially compensate for various optical system maladies. In more recent incarnations, OPC is said to have evolved into compensation to ensure design pattern fidelity which takes into account other processing steps such as wafer etch effects. The nonprinting assist features mentioned are with respect to a fine reposition or modulation of a feature's edge so that the printed dimension is closer to the intended dimension. Examples of OPC structures are illustrated in FIG. 7 and in more detail in FIGS. 8a through 8c of the Komblit et al. article, for example, where the pattern imaged in the photoresist is shown in FIG. 8, and the pattern as wet or dry etched into the reticle pattern transfer layer are shown in FIGS. 8a through 8c. A photoresist used to pattern a reticle may make use of assist features, which subsequently appear as part of the patterned reticle, although there may be some change in the dimensions of the assist features as a result of the processing of the reticle. When the reticle is used to transfer the pattern to a photoresist material on the surface of a semiconductor substrate, the assist features typically do not appear at all in the patterned photoresist material or only minor artifacts appear. The Kornblit et al. article shows this in FIGS. 9a through 9c, which represent the wafer prints produced using the masks shown in FIGS. 8a through 8c. The Kornblit et al. article FIG. 8 showing the DUV data is shown as pattern 100 in FIG. 1A herein; the FIG. 8a which shows the dry etched reticle (mask) pattern layer 120 generated from the FIG. 8 pattern is shown as FIG. 1B herein; and the FIG. 9a illustration of the pattern transferred 130 into the semiconductor wafer is shown as FIG. 1C herein. The dry etched reticle Figures pertain to 4×250 nm pattern generation, i.e. a reticle pattern feature size of 1,000 nm, which translates to a 250 nm feature size on the semiconductor wafer.
Typically, for optical imaging of photoresists, the assist features are typically less than about 0.5 times the size of the target feature, in a line corner extender, for example. Currently the smallest feature which can be written using an optical continuous wave laser writing tool of the kind described herein is about 175 nm; however, at present there is no process window for this feature size which would enable a manufacturing process. The assist feature (optical proximity correction structure/OPC) for a 360 nm (manufacturable) feature size would be in the range of about 180 nm. The assist feature used at the corner of a line might then be a square 102, such as those shown at the corners of a line 104, which is presented in FIG. 1 in the present application. The square 102 would have a side dimension 106 of about 180 nm. Although it is possible to write a 180 nm latent image in the photoresist, for example a square which is 180 nm on a side, this assist feature is sufficiently small that it typically cannot be properly imaged (printed) using optical imaging techniques.
To extend the range of the optically-generated reticle to feature sizes which are below about 400 nm on the reticle, for example, there is a need for improvement in the developed photoresist patterns which are translated into the reticle pattern.
The present invention relates to improving the reticle processing window in a manner which enables patterning of smaller dimension features, and which enables better resolution of these smaller dimension features. In particular, the invention relates to an improvement in process bias and pattern resolution during the patterning of the photoresist mask which is used to transfer the pattern to the reticle.